The goal of this project is to investigate whether particular toxicants, including some putative endocrine disrupting chemicals, can increase intracellular estradiol concentrations by inhibiting estradiol metabolism and depuration, and thus increasing estradiol-responsive growth. Further, the role of the Pregnane X-receptor (PXR) and its ligands in regulating estradiol metabolism and elimination will be investigated. Steroid hormone metabolism and depuration is controlled by the same enzymes that metabolize and eliminate toxicants in phase I, II and III reactions. Perturbing these metabolic processes can result in toxicant interactions, as well as, perturb steroid hormone homeostasis. MCF-7 breast cancer cells will be stably transfected with the PXR receptor and putative PXR ligands, such as 4-nonylphenol, DEHP (antagonists) and rifampicin (agonist) will be used to determine whether PXR ligands can significantly alter intracellular estradiol homeostasis and increase estrogen-responsive proliferation. The PXR regulates induction of CYP3A and MRP2 and may be involved in the induction of glucuronosyltransferases, as well. CYP3A is a major steroid hydroxylase, glucuronosyltransferases are important in conjugating steroids and MRP2 is a steroid hormone transporter. Hence, these enzymes are important in metabolizing and eliminating estradiol. Recent evidence in our lab and others suggests that some chemicals may bind the PXR and act as antagonists. Our hypothesis is that these chemicals, including the endocrine disrupters, DEHP and 4-nonylphenol, can down-regulate estradiol metabolism and depuration in this manner, thus increase effective estrogen concentrations and induce proliferation. This alternate mechanism of endocrine disruption could cause developmental abnormalities and/or cause estrogen-responsive cancers, such as breast and ovarian cancer.